Methods for controlling mass flow rates and pressures in passageways coupled to reaction chambers and systems for depositing material onto microfeature workpieces in reaction chambers

ABSTRACT

Methods, apparatuses, and systems for controlling mass flow rates and pressures in passageways coupled to reaction chambers are disclosed herein. In one embodiment, a method includes controlling a mass flow rate in a passageway in response to a first condition by modulating a valve of a mass flow and pressure control unit, and controlling a pressure in the passageway in response to a second condition different than the first condition by modulating the valve of the mass flow and pressure control unit. In another embodiment, an apparatus includes a mass flow measurement device, a pressure sensor, a modulating valve in the passageway, and a controller operably coupled to the mass flow measurement device, the pressure sensor, and the modulating valve. The controller has a computer-readable medium containing instructions to perform the above-mentioned method.

TECHNICAL FIELD

[0001] The present invention is related to methods for controlling mass flow rates and pressures in passageways coupled to reaction chambers, and systems for depositing material in thin film deposition processes used in the manufacturing of microfeatures.

BACKGROUND

[0002] Thin film deposition techniques are widely used in the manufacturing of microfeatures to form a coating on a workpiece that closely conforms to the surface topography. The size of the individual components in the devices is constantly decreasing, and the number of layers in the devices is increasing. As a result, the density of components and the aspect ratios of depressions (i.e., the ratio of the depth to the size of the opening) are increasing. The size of workpieces is also increasing to provide more real estate for forming more dies (i.e., chips) on a single workpiece. Many fabricators, for example, are transitioning from 200 mm to 300 mm workpieces, and even larger workpieces will likely be used in the future. Thin film deposition techniques accordingly strive to produce highly uniform conformal layers that cover the sidewalls, bottoms, and corners in deep depressions that have very small openings.

[0003] One widely used thin film deposition technique is Chemical Vapor Deposition (CVD). In a CVD system, one or more precursors that are capable of reacting to form a solid thin film are mixed in a gas or vapor state, and then the precursor mixture is presented to the surface of the workpiece. The surface of the workpiece catalyzes the reaction between the precursors to form a solid thin film at the workpiece surface. A common way to catalyze the reaction at the surface of the workpiece is to heat the workpiece to a temperature that causes the reaction.

[0004] Although CVD techniques are useful in many applications, they also have several drawbacks. For example, if the precursors are not highly reactive, then a high workpiece temperature is needed to achieve a reasonable deposition rate. Such high temperatures are not typically desirable because heating the workpiece can be detrimental to the structures and other materials already formed on the workpiece. Implanted or doped materials, for example, can migrate within the silicon substrate at higher temperatures. On the other hand, if more reactive precursors are used so that the workpiece temperature can be lower, then reactions may occur prematurely in the gas phase before reaching the substrate. This is undesirable because the film quality and uniformity may suffer, and also because it limits the types of precursors that can be used.

[0005] Atomic Layer Deposition (ALD) is another thin film deposition technique. FIGS. 1A and 1B schematically illustrate the basic operation of ALD processes. Referring to FIG. 1A, a layer of gas molecules A, coats the surface of a workpiece W. The layer of A_(x) molecules is formed by exposing the workpiece W to a precursor gas containing A_(x) molecules and then purging the chamber with a purge gas to remove excess A_(x) molecules. This process can form a monolayer of A_(x) molecules on the surface of the workpiece W because the A_(x) molecules at the surface are held in place during the purge cycle by physical adsorption forces at moderate temperatures or chemisorption forces at higher temperatures. Referring to FIG. 1B, the layer of A_(x) molecules is then exposed to another precursor gas containing B_(y) molecules. The A_(x) molecules react with the B_(y) molecules to form an extremely thin layer of solid material on the workpiece W. The chamber is then purged again with a purge gas to remove excess B_(y) molecules.

[0006]FIG. 2 illustrates the stages of one cycle for forming a thin solid layer using ALD techniques. A typical cycle includes (a) exposing the workpiece to the first precursor A_(x), (b) purging excess A_(x) molecules, (c) exposing the workpiece to the second precursor B_(y), and then (d) purging excess B_(y) molecules. In actual processing, several cycles are repeated to build a thin film on a workpiece having the desired thickness. For example, each cycle may form a layer having a thickness of approximately 0.5-1.0 Å, and thus it takes approximately 60-120 cycles to form a solid layer having a thickness of approximately 60 Å.

[0007]FIG. 3 schematically illustrates a single-wafer ALD reactor 10 having a reaction chamber 20 coupled to a gas supply 30 and a vacuum 40. The reactor 10 also includes a heater 50 that supports the workpiece W and a gas dispenser 60 in the reaction chamber 20. The gas dispenser 60 includes a plenum 62 operably coupled to the gas supply 30 and a distributor plate 70 having a plurality of holes 72. In operation, the heater 50 heats the workpiece W to a desired temperature, and the gas supply 30 selectively injects the first precursor A_(x), the purge gas, and the second precursor B_(y) as shown above in FIG. 2. The vacuum 40 maintains a negative pressure in the chamber to draw the gases from the gas dispenser 60 across the workpiece W and then through an outlet of the reaction chamber 20.

[0008] One drawback of ALD processing is that it has a relatively low throughput compared to CVD techniques. For example, each A_(x)-purge-B_(y)-purge cycle can take several seconds. This results in a total process time of several minutes to form a single thin layer of only 60 Å. In contrast to ALD processing, CVD techniques require only about one minute to form a 60 Å thick layer. The low throughput of existing ALD techniques limits the utility of the technology in its current state because ALD may be a bottleneck in the overall manufacturing process.

[0009] Another drawback of ALD and pulsed CVD processing is that it is difficult to control the amount of gas in the chamber at the beginning of a pulse. In ALD and CVD processing, a mass flow controller or a pressure controller typically controls the flow of precursors into the reactor. For example, when a mass flow controller controls the gas flow, the beginning of the pulse can have an undesirably high flow rate. Mass flow controllers are susceptible to producing high initial flow rates because they continue to cause the gas to flow through the gas line even when a final valve in the supply line is closed to prevent the precursor from flowing into the reactor. This causes the pressure in the supply line to continually increase. Moreover, as the flow rate drops when the final valve is closed, the mass flow controller causes an upstream valve to open more. As such, when the final valve opens, the increased pressure in the supply line and the open upstream valve creates a large inrush of precursors into the reaction chamber. This can cause premature reactions between the precursors or remove molecules from the surface of the workpiece W, which can result in a nonuniform surface on the workpiece W. Furthermore, the increased pressure in the supply line can cause the final valve to dispense too much precursor into the reaction chamber, which can make it difficult to purge or otherwise remove the excess precursor from the reaction chamber.

[0010] Systems that use pressure controllers to control the gas flow also have drawbacks. For example, the pressure controller typically has a fixed or adjustable orifice in the gas supply line to limit the precursor flow when the pressure drops downstream. The pressure controller, however, cannot control the mass flow rate, and consequently, differences in the downstream pressure can cause different mass flow rates in the gas supply line. Accordingly, too much or too little precursor may be dispensed into the reaction chamber in systems that rely on pressure controllers to modulate the gas flow into the reactor. Thus, there is a need to improve the process of providing precursors to reactors.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIGS. 1A and 1B are schematic cross-sectional views of stages in ALD processing in accordance with the prior art.

[0012]FIG. 2 is a graph illustrating a cycle for forming a layer using ALD techniques in accordance with the prior art.

[0013]FIG. 3 is a schematic representation of a system including a reactor for depositing material onto a microfeature workpiece in accordance with the prior art.

[0014]FIG. 4 is a schematic representation of a system including a plurality of mass flow and pressure control units, and a reactor for depositing material onto a microfeature workpiece in accordance with one embodiment of the invention.

[0015]FIG. 5 is a schematic representation of an embodiment of a mass flow and pressure control unit for use in the system shown in FIG. 4.

[0016]FIG. 6 is a flow diagram illustrating one method of prioritizing between regulating the mass flow rate and regulating the pressure.

[0017]FIG. 7 is a flow diagram illustrating another method of prioritizing between regulating the mass flow rate and regulating the pressure.

DETAILED DESCRIPTION

[0018] A. Overview

[0019] The following disclosure describes several embodiments of mass flow and pressure control units, systems including mass flow and pressure control units, and methods for controlling mass flow rates and pressures in passageways. Many specific details of the invention are described below with reference to single-wafer reactors for depositing materials onto microfeature workpieces, but several embodiments can be used in batch systems for processing a plurality of workpieces simultaneously. The term “microfeature workpiece” is used throughout to include substrates upon which and/or in which microelectronic devices, micromechanical devices, data storage elements, read/write components, and other features are fabricated. For example, microfeature workpieces can be semiconductor wafers such as silicon or gallium arsenide wafers, glass substrates, insulative substrates, and many other types of materials. Furthermore, the term “gas” is used throughout to include any form of matter that has no fixed shape and will conform in volume to the space available, which specifically includes vapors (i.e., a gas having a temperature less than the critical temperature so that it may be liquefied or solidified by compression at a constant temperature). Several embodiments in accordance with the invention are set forth in FIGS. 4-7 and the following text to provide a thorough understanding of particular embodiments of the invention. A person skilled in the art will understand, however, that the invention may have additional embodiments, or that the invention may be practiced without several of the details of the embodiments shown in FIGS. 4-7.

[0020] One aspect of the invention is directed to a method for controlling a mass flow rate and a pressure in a passageway. In one embodiment, the method includes controlling a mass flow rate in a passageway in response to a first condition by modulating a valve of a mass flow and pressure control unit. The method further includes controlling a pressure in the passageway in response to a second condition by modulating the valve of the mass flow and pressure control unit. In one aspect of this embodiment, the method can further include determining the pressure in the passageway and determining the mass flow rate in the passageway. The first condition can include the determined mass flow rate in the passageway and the second condition can include the determined pressure in the passageway.

[0021] In another embodiment of the invention, the method includes regulating a first valve of a mass flow and pressure control unit to control a pressure in a passageway when a second valve in the passageway is in a first position. The method further includes regulating the first valve of the mass flow and pressure control unit to control a mass flow rate in the passageway when the second valve is in a second position. The second valve is downstream from the mass flow and pressure control unit, and the first and second positions can be closed or at least partially open positions. In one aspect of this embodiment, regulating the first valve can include modulating the first valve to control the mass flow rate when the second valve is in the at least partially open position and when the determined pressure in the passageway is less than or equal to a predetermined pressure. In another aspect of this embodiment, regulating the first valve to control the pressure includes modulating the first valve in response to a determined pressure in the passageway, and regulating the first valve to control the mass flow rate includes modulating the first valve in response to a determined mass flow rate.

[0022] Another aspect of the invention is directed to a mass flow and pressure control apparatus for controlling a mass flow rate and a pressure in a conduit with a passageway. The apparatus includes a mass flow measurement device coupled to the conduit to determine the mass flow rate in the passageway, a pressure sensor coupled to the conduit to determine the pressure in the passageway, a modulating valve in the passageway, and a controller operably coupled to the mass flow measurement device, the pressure sensor, and the modulating valve. The controller has a computer-readable medium containing instructions to perform any one of the above-mentioned methods.

[0023] B. Deposition Systems

[0024]FIG. 4 is a schematic representation of a system 100 for depositing material onto a microfeature workpiece W in accordance with one embodiment of the invention. In this embodiment, the system 100 includes a reactor 110 having a reaction chamber 120 coupled to a gas supply 130 and a vacuum 140. The reactor 110 also includes a gas distributor 160 coupled to the reaction chamber 120 and the gas supply 130 to dispense the gas(es) into the reaction chamber 120 and onto the workpiece W.

[0025] The gas supply 130 includes a plurality of gas sources 132 (identified individually as 132 a-c) and a plurality of first gas conduits 136 coupled to the gas sources 132. The gas sources 132 can include a first gas source 132 a for providing a first gas, a second gas source 132 b for providing a second gas, and a third gas source 132 c for providing a third gas. The first and second gases can be first and second precursors, respectively. The third gas can be a purge gas. The first and second precursors are the gas or vapor phase constituents that react to form the thin, solid layer on the workpiece W. The purge gas can be a suitable type of gas that is compatible with the reaction chamber 120 and the workpiece W. In other embodiments, the gas supply 130 can include a different number of gas sources 132 for applications that require additional precursors or purge gases. In additional embodiments, the gas sources 132 can include one or more etchants for deposition onto a microfeature workpiece during etching.

[0026] The system 100 of the illustrated embodiment also includes a plurality of mass flow and pressure control (MFPC) units 170 (shown schematically) coupled to the first gas conduits 136, a plurality of second gas conduits 137 coupled to the MFPC units 170, and a valve assembly 133 coupled to the second gas conduits 137. The MFPC units 170 are operated by a controller 142 that generates signals for controlling the mass flow rate and the pressure of the gas(es) in the second gas conduits 137, as described in greater detail below. The valve assembly 133 is also operated by the controller 142 to control the flow of gases through the reaction chamber 120 for ALD and CVD applications. For example, the controller 142 can be programmed to operate the valve assembly 133 to pulse the gases individually through the gas distributor 160 in ALD applications or mix selected precursors in the gas distributor 160 in CVD applications. In additional embodiments, the system 100 can have other configurations. For example, several gas conduits can be coupled to a single MFPC unit. In this embodiment, a MFPC unit could be coupled to a first conduit carrying a precursor and a second conduit carrying a purge gas.

[0027] In the illustrated embodiment, the reactor 110 also includes a workpiece support 150 to hold the workpiece W in the reaction chamber 120. In one aspect of this embodiment, the workpiece support 150 can be heated to bring the workpiece W to a desired temperature for catalyzing the reaction between the first gas and the second gas at the surface of the workpiece W. For example, the workpiece support 150 can be a plate with a heating element. The workpiece support 150, however, may not be heated in other applications.

[0028] C. Mass Flow and Pressure Control Units

[0029]FIG. 5 is a schematic representation of an embodiment of a MFPC unit 170 for use in the system 100 shown in FIG. 4. The MFPC unit 170 controls the mass flow rate and the pressure of the gas to provide the desired amount of gas to the reactor 110 (FIG. 4) at the desired pressure. The MFPC unit 170 is coupled to the first and second gas conduits 136 and 137, which each include a passageway 138. The gas flows in the direction D through the passageway 138 from the first gas conduit 136, through the MFPC unit 170, and into the second gas conduit 137. The MFPC unit 170 controls both the mass flow rate and pressure of the gas in the second gas conduit 137.

[0030] The MFPC unit 170 includes a mass flow measurement device 180 in fluid communication with the passageway 138 to determine the mass flow through the passageway 138. In the illustrated embodiment, the mass flow measurement device 180 includes a laminar flow section 190 in the passageway 138 and a bypass 184 around the laminar flow section 190 of the passageway 138. The laminar flow section 190 can include a plurality of flow tubes or other devices to create a laminar flow of the gas. The laminar flow section 190 causes the ratio of the gas D₁ flowing through the laminar flow section 190 to the gas D₂ flowing through the bypass 184 to remain constant.

[0031] The bypass 184 includes a first temperature element 182 a electrically coupled to the controller 142, a second temperature element 182 b electrically coupled to the controller 142, an inlet 186, and an outlet 188. The first and second temperature elements 182 a-b measure the temperature change in the gas as it flows through a portion of the bypass 184. For example, in one embodiment, the bypass 184 can include a tube 185, and the first and second temperature elements 182 a-b can include resistance thermometer elements that are wound around the tube 185. The tube 185 can be heated by applying an electrical current to the resistance thermometer elements. The gas cools as it flows through the tube 185 and thus creates a temperature difference between the two resistance thermometer elements. The temperature difference causes a change in the resistance of the thermometer elements that can be measured as an electrical signal. The temperature difference between the first and second temperature elements 182 a-b is dependent on the mass flow of the gas and is a function of the gas's density, specific heat, and flow rate. Accordingly, the controller 142 can calculate the mass flow of the gas in the passageway 138 from the temperature difference between the first and second temperature elements 182 a-b. In other embodiments, the temperature elements 182 can include thermal couples or other suitable devices to determine the temperature of the gas in the bypass 184, or the mass flow measurement device 180 can determine the mass flow of the gas through the passageway 138 with other methods or devices.

[0032] The MFPC unit 170 further includes a first pressure sensor 172 that is operably coupled to the controller 142 to determine the pressure in the passageway 138 of the second gas conduit 137. The first pressure sensor 172 can be a Baratron® capacitance manometer manufactured by MKS Instruments, Inc. of Andover, Mass., or any other suitable device. In some embodiments, the MFPC unit 170 can also include a second pressure sensor 174 (shown in hidden lines) that measures the pressure in the passageway 138 of the first gas conduit 136. In other embodiments, the MFPC unit 170 may not include the second pressure sensor 174.

[0033] The MFPC unit 170 also includes a modulating valve 176 that is positioned in the passageway 138 to regulate the flow of gas between the first and second gas conduits 136 and 137. The controller 142 is operably coupled to the modulating valve 176 to selectively control the position of the valve 176 and accordingly the gas flow through the valve 176. The modulating valve 176 can be driven by a piezoelectric, solenoid, thermal, or other suitable actuator. The controller 142 is also operably coupled to a valve 133 a of the valve assembly 133 (FIG. 4). The valve 133 a controls the flow of gas from the second gas conduit 137 into the reaction chamber 120 (FIG. 4).

[0034] In operation, in the illustrated embodiment, the first and second pressure sensors 172 and 174, the first and second temperature elements 182 a-b, and the valve 133 a provide electrical signals to the controller 142 as inputs. Based on at least one of these inputs, a predetermined mass flow rate setpoint, and/or a predetermined pressure setpoint, the controller 142 provides an electrical signal as an output to the modulating valve 176 indicating the proper position of the modulating valve 176. The modulating valve 176 responds accordingly to restrict or open the passageway 138 and thereby control the mass flow rate or pressure in the second gas conduit 137.

[0035] The controller 142 can be programmed to prioritize between regulating the mass flow rate and regulating the pressure. FIG. 6 is a flow diagram illustrating one method of prioritizing between regulating the mass flow rate and regulating the pressure. Referring to FIGS. 5 and 6, in one embodiment, such as in an ALD or CVD application, the position of the valve 133 a determines whether the controller 142 regulates the mass flow rate or the pressure. For example, when the valve 133 a is closed and the gas is charged in the second gas conduit 137, the controller 142 can control the pressure of the gas in the second gas conduit 137. More specifically, the second gas conduit 137 can be pressurized to the predetermined pressure setpoint. Accordingly, the controller 142 opens the modulating valve 176 to allow gas to flow into the second gas conduit 137 until the pressure in the conduit 137 reaches the pressure setpoint. When the pressure in the second gas conduit 137 reaches the pressure setpoint, the controller 142 closes the modulating valve 176 to limit the pressure in the second gas conduit 137. When the valve 133 a opens to deliver the pressurized gas into the reaction chamber 120 (FIG. 4), the controller 142 can control the mass flow rate of the gas flowing into the second gas conduit 137 by regulating the modulating valve 176. For example, the controller 142 can open the modulating valve 176 until the mass flow rate reaches the mass flow rate setpoint.

[0036] In one aspect of this embodiment, when the valve 133 a is open the controller 142 can control the pressure instead of the mass flow rate if the pressure in the second gas conduit 137 exceeds the pressure setpoint. When the valve 133 a is subsequently closed, the controller 142 can continue to control the pressure in the second gas conduit 137 as described above. In additional embodiments, the controller 142 can control the pressure when the valve 133 a is open for reasons other than those described above. For example, the controller 142 can control the pressure when the reaction chamber 120 (FIG. 4) is being purged.

[0037]FIG. 7 is a flow diagram illustrating another method of prioritizing between regulating the mass flow rate and regulating the pressure. Referring to FIGS. 5 and 7, the controller 142 can prioritize between regulating the mass flow rate and regulating the pressure based on the pressure in the second gas conduit 137. In this embodiment, the controller 142 can regulate the modulating valve 176 to control the mass flow rate through the passageway 138 until the pressure in the second gas conduit 137 exceeds the pressure setpoint. When the pressure in the second gas conduit 137 exceeds the pressure setpoint, the controller 142 regulates the modulating valve 176 to limit the pressure in the second gas conduit 137 to the pressure setpoint. As soon as the pressure in the second gas conduit 137 falls below the pressure setpoint, the controller 142 can revert back to controlling the mass flow rate in the passageway 138.

[0038] In another embodiment of the invention, the controller 142 can prioritize between regulating the mass flow rate and regulating the pressure based on the pressure in the first gas conduit 136 as measured by the second pressure sensor 174. The second pressure sensor 174 measures the pressure upstream of the modulating valve 176 and thus enables the controller 142 to anticipate changes to the pressure in the second gas conduit 137. In other embodiments, the controller 142 can prioritize between controlling the mass flow rate and controlling the pressure based on other factors.

[0039] Referring back to FIG. 5, in additional embodiments, the system 100 may include a valve 133 b (shown in hidden lines) between the valve 133 a and the MFPC unit 170 to divert gas flow into a bypass in order to purge the first and second gas conduits 136 and 137. In such an embodiment, the controller 142 could control the mass flow rate of the purge gas in the passageway 138.

[0040] One advantage of the MFPC units of the illustrated embodiments is the ability to control both the mass flow rate and pressure of a gas flowing into a reaction chamber. For example, the ability to control the pressure in the gas conduit prevents a large inrush of gas into the reactor when the valve to the reaction chamber is opened. The large inrush of gas can cause premature reactions between precursors, remove material from the surface of the workpiece, and result in too much gas being dispensed into the reaction chamber. The ability to control the mass flow rate allows the controller to ensure the reaction chamber receives the proper amount of gas.

[0041] From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration but that various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims. 

I/We claim:
 1. A method for controlling a vapor deposition process in the fabrication of microdevices on microfeature workpieces, the method comprising: regulating a first valve of a mass flow and pressure control unit to control a pressure in a passageway when a second valve in the passageway is in a first position, the second valve being downstream from the mass flow and pressure control unit; and regulating the first valve of the mass flow and pressure control unit to control a mass flow rate in the passageway when the second valve is in a second position different than the first position.
 2. The method of claim 1 wherein the first position is a closed position, and wherein regulating the first valve to control the pressure comprises modulating the first valve to control the pressure when the second valve is in the closed position.
 3. The method of claim 1 wherein the second position is an at least partially open position, and wherein regulating the first valve to control the mass flow rate comprises modulating the first valve to control the mass flow rate when the second valve is in the at least partially open position.
 4. The method of claim 1, further comprising: determining the pressure in the passageway; wherein the second position is an at least partially open position; and wherein regulating the first valve to control the mass flow rate comprises modulating the first valve to control the mass flow rate when the second valve is in the at least partially open position and when the determined pressure in the passageway is less than or equal to a predetermined pressure.
 5. The method of claim 1, further comprising: determining the pressure in the passageway; and determining the mass flow rate in the passageway; wherein regulating the first valve to control the pressure comprises modulating the first valve in response to the determined pressure in the passageway; and wherein regulating the first valve to control the mass flow rate comprises modulating the first valve in response to the determined mass flow rate in the passageway.
 6. The method of claim 1, further comprising: determining the pressure in the passageway downstream from the first valve; and determining the pressure in the passageway upstream from the first valve; wherein regulating the first valve to control the pressure comprises modulating the first valve to control the pressure in response to the determined pressure in the passageway downstream and/or upstream from the first valve.
 7. The method of claim 1 wherein: regulating the first valve to control the pressure comprises modulating the first valve to maintain the pressure in the passageway at a predetermined pressure; and regulating the first valve to control the mass flow rate comprises modulating the first valve to maintain the mass flow rate at a predetermined mass flow rate.
 8. The method of claim 1 wherein regulating the first valve to control the mass flow rate comprises controlling the mass flow rate as a gas flows through the passageway and into the reaction chamber.
 9. The method of claim 1 wherein regulating the first valve to control the pressure comprises controlling the pressure in the passageway when the passageway is charged with a gas.
 10. The method of claim 1 wherein the mass flow and pressure control unit further comprises a mass flow measurement device operably coupled to the passageway, a pressure sensor operably coupled to the passageway, and a controller operably coupled to the pressure sensor and the mass flow measurement device.
 11. A method for controlling a vapor deposition process in the fabrication of microdevices on microfeature workpieces, the method comprising: controlling a mass flow rate in a passageway in response to a first condition by modulating a valve of a mass flow and pressure control unit; and controlling a pressure in the passageway in response to a second condition different than the first condition by modulating the valve of the mass flow and pressure control unit.
 12. The method of claim 11 wherein: the valve comprises a first valve; the first condition comprises a second valve being in an at least partially open position, the second valve being in the passageway downstream from the mass flow and pressure control unit; and controlling the mass flow rate in the passageway comprises controlling the mass flow rate in response to the second valve being in the at least partially open position.
 13. The method of claim 11 wherein: the valve comprises a first valve; the second condition comprises a second valve being in a closed position, the second valve being in the passageway downstream from the mass flow and pressure control unit; and controlling the pressure in the passageway comprises controlling the pressure in response to the second valve being in the closed position.
 14. The method of claim 11, further comprising: determining the pressure in the passageway; wherein the valve comprises a first valve; wherein the first condition comprises a second valve being in an at least partially open position and the determined pressure in the passageway being less than or equal to a predetermined pressure, the second valve being in the passageway downstream from the mass flow and pressure control unit; and wherein controlling the mass flow rate in the passageway comprises controlling the mass flow rate in response to the second valve being in the at least partially open position and the determined pressure in the passageway being less than or equal to the predetermined pressure.
 15. The method of claim 11, further comprising: determining the pressure in the passageway; wherein the valve comprises a first valve; wherein the second condition comprises a second valve being in an at least partially open position and the determined pressure in the passageway being greater than a predetermined pressure, the second valve being in the passageway downstream from the mass flow and pressure control unit; and wherein controlling the pressure in the passageway comprises controlling the pressure in response to the second valve being in the at least partially open position and the determined pressure in the passageway being greater than the predetermined pressure.
 16. The method of claim 11, further comprising: determining the pressure in the passageway; and determining the mass flow rate in the passageway; wherein the first condition comprises the determined mass flow rate in the passageway and the second condition comprises the determined pressure in the passageway; wherein controlling the mass flow rate comprises controlling the mass flow rate in response to the determined mass flow rate in the passageway; and wherein controlling the pressure comprises controlling the pressure in response to the determined pressure in the passageway.
 17. The method of claim 11, further comprising: determining the pressure in the passageway; wherein the second condition comprises the determined pressure in the passageway exceeding a predetermined pressure; and wherein controlling the pressure comprises controlling the pressure in the passageway in response to the determined pressure in the passageway exceeding the predetermined pressure.
 18. The method of claim 11, further comprising: determining the pressure in the passageway downstream of the valve; and determining the pressure in the passageway upstream of the valve; wherein the second condition comprises the determined pressure in the passageway upstream and/or downstream of the valve; and wherein controlling the pressure comprises controlling the pressure in the passageway in response to the determined pressure in the passageway upstream and/or downstream of the valve.
 19. The method of claim 11 wherein: controlling the mass flow rate comprises limiting the mass flow rate in the passageway to a predetermined mass flow rate; and controlling the pressure comprises limiting the pressure in the passageway to a predetermined pressure.
 20. The method of claim 11 wherein controlling the mass flow rate comprises controlling the mass flow rate as a gas flows through the passageway and into a reaction chamber coupled to the passageway.
 21. The method of claim 11 wherein controlling the pressure comprises controlling the pressure in the passageway when the passageway is charged with a gas.
 22. The method of claim 11 wherein the mass flow and pressure control unit further comprises a mass flow measurement device operably coupled to the passageway, a pressure sensor operably coupled to the passageway, and a controller operably coupled to the pressure sensor and the mass flow measurement device.
 23. A method for controlling a vapor deposition process in the fabrication of microdevices on microfeature workpieces, the method comprising: determining a pressure in a passageway; determining a mass flow rate in the passageway; and regulating a valve of a mass flow and pressure control unit to control at least one of the pressure and the mass flow rate in the passageway in response to at least one of the determined pressure and the determined mass flow rate.
 24. The method of claim 23 wherein the valve comprises a first valve, and wherein the method further comprises: opening at least partially a second valve downstream from the mass flow and pressure control unit; and flowing a gas from the passageway into a reaction chamber while regulating the valve to control the mass flow rate in the passageway.
 25. The method of claim 23, further comprising prioritizing between regulating the valve of the mass flow and pressure control unit to control the pressure and regulating the valve to control the mass flow rate.
 26. The method of claim 23, further comprising prioritizing between regulating the valve of the mass flow and pressure control unit to control the pressure and regulating the valve to control the mass flow rate based on a predetermined pressure and a predetermined mass flow rate.
 27. The method of claim 23 wherein the valve of the mass flow and pressure control unit comprises a first valve, and wherein the method further comprises prioritizing between regulating the valve to control the pressure and regulating the valve to control the mass flow rate based on a position of a second valve downstream from the mass flow and pressure control unit, the second valve being movable between an open position and a closed position.
 28. The method of claim 23 wherein: determining the pressure comprises determining the pressure in the passageway upstream of the valve; and regulating the valve to control the pressure comprises regulating the valve in response to the determined upstream pressure.
 29. The method of claim 23 wherein regulating the valve to control the mass flow rate comprises controlling the mass flow rate of a gas flowing from the passageway into a reaction chamber.
 30. The method of claim 23 wherein regulating the valve to control the pressure comprises controlling the pressure in the passageway as the passageway is charged with a gas.
 31. The method of claim 23 wherein the mass flow and pressure control unit further comprises a mass flow measurement device operably coupled to the passageway, a pressure sensor operably coupled to the passageway, and a controller operably coupled to the pressure sensor and the mass flow measurement device.
 32. A mass flow and pressure control apparatus for controlling a mass flow rate and a pressure in a conduit with a passageway, the apparatus comprising: a mass flow measurement device coupled to the conduit to determine the mass flow rate in the passageway; a pressure sensor coupled to the conduit to determine the pressure in the passageway; a modulating valve in the passageway; and a controller operably coupled to the mass flow measurement device, the pressure sensor, and the modulating valve, the controller having a computer-readable medium containing instructions to perform a method comprising controlling the mass flow rate in the passageway in response to a first condition by regulating the modulating valve; and controlling the pressure in the passageway in response to a second condition different than the first condition by regulating the modulating valve.
 33. The apparatus of claim 32 wherein the pressure sensor comprises a first pressure sensor positioned downstream from the modulating valve, and wherein the apparatus further comprises a second pressure sensor coupled to the conduit and positioned upstream from the modulating valve, the second pressure sensor being operably coupled to the controller.
 34. The apparatus of claim 32 wherein the computer-readable medium contains instructions to prioritize between controlling the mass flow rate and controlling the pressure.
 35. The apparatus of claim 32 wherein: the modulating valve comprises a first valve; the first condition comprises a second valve being in an at least partially open position, the second valve being in the passageway downstream from the first valve; and the second condition comprises the second valve being in a closed position.
 36. A mass flow and pressure control apparatus for controlling a mass flow rate and a pressure in a conduit with a passageway, the apparatus comprising: a mass flow measurement device coupled to the conduit to determine the mass flow rate in the passageway; a pressure sensor coupled to the conduit to determine the pressure in the passageway; a modulating valve in the passageway; and a controller operably coupled to the mass flow measurement device, the pressure sensor, and the modulating valve, the controller having a computer-readable medium containing instructions to perform a method comprising determining the pressure in the passageway; determining the mass flow rate in the passageway; and regulating the modulating valve to control at least one of the pressure and the mass flow rate in the passageway in response to at least one of the determined pressure and the determined mass flow rate.
 37. The apparatus of claim 36 wherein the pressure sensor comprises a first pressure sensor positioned downstream from the modulating valve, and wherein the apparatus further comprises a second pressure sensor coupled to the conduit and positioned upstream from the modulating valve, the second pressure sensor being operably coupled to the controller.
 38. The apparatus of claim 36 wherein the computer-readable medium contains instructions to prioritize between controlling the mass flow rate and controlling the pressure.
 39. A system for depositing material onto a surface of a microfeature workpiece in a reaction chamber, the system comprising: a gas conduit having a gas passageway; a gas supply assembly coupled to the gas conduit, the gas supply assembly having a gas source; a reaction chamber; a gas distributor carried by the reaction chamber and coupled to the gas conduit; and a mass flow and pressure control unit comprising a mass flow measurement device coupled to the gas conduit to determine the mass flow in the gas passageway; a pressure sensor coupled to the gas conduit to determine the pressure in the gas passageway; a modulating valve in the gas passageway; and a controller operably coupled to the mass flow measurement device, the pressure sensor, and the modulating valve, the controller configured to regulate the modulating valve to control at least one of the mass flow rate and the pressure in the gas passageway.
 40. The system of claim 39 wherein the controller is configured to regulate the modulating valve in response to a first input from the mass flow measurement device and/or a second input from the pressure sensor.
 41. The system of claim 39 wherein the controller is configured to prioritize between regulating the modulating valve to control the mass flow rate and regulating the modulating valve to control the pressure.
 42. The system of claim 39 wherein the pressure sensor comprises a first pressure sensor positioned downstream from the modulating valve, and wherein the mass flow and pressure control unit further comprises a second pressure sensor coupled to the gas conduit and positioned upstream from the modulating valve.
 43. The system of claim 39 wherein the modulating valve comprises a first modulating valve, wherein the system further comprises a second valve in the gas passageway downstream from the first valve, the second valve being movable between an open position and a closed position, and wherein the controller is configured to regulate the first modulating valve in response to the position of the second valve. 